Under standard operating conditions, automotive engines generate undesirable gases such as carbon monoxide (CO), hydrocarbons and nitrogen oxides (NOx). In order to reduce the emission of these gases into the atmosphere, automobile manufacturers have for a number of years, employed catalysts that facilitate the conversion of these substances into less noxious compounds.
There are many different types of catalysts, and the different types of catalysts may be used individually or in certain circumstances in combination. The selection of the type of catalyst or catalysts to use in a given application, in part, depends upon the conditions under which the engine in which it will be used will operate.
An automotive engine can operate under either rich or lean conditions or under alternating periods of rich and lean conditions. A rich operating condition refers to a condition in which there is a normalized air: fuel ratio of less than 1. By contrast, a lean operating condition refers to a condition in which there is a normalized air: fuel ratio of greater than 1. Diesel engines, for example, typically operate under lean conditions. By contrast, gasoline engines typically operate under stoichiometric conditions, i.e. the normalized air:fuel ratio is approximately 1. The concepts of rich and lean operating conditions are well known to persons skilled in the art.
One type of catalyst that is used in automotive engines and that is well known to persons skilled in the art is the three-way catalyst. Three-way catalysts, when operated under stoichiometric conditions are efficient at treating CO, hydrocarbons and NOx. Under these conditions, a three-way catalyst can efficiently convert CO, hydrocarbons and NOx, into CO2, H2O and N2. However, although three-way catalysts are efficient at treating all of the aforementioned substances under stoichiometric conditions, under lean conditions, they are not efficient at converting NOx into N2.
There are certain NOx catalysts that can treat nitrogen oxides under lean conditions. These types of catalysts include metal ion-exchanged zeolite materials, which are also well known to persons skilled in the art. In these types of catalysts, there is selective reduction of NOx to N2. However, metal ion-exchanged zeolite materials are only moderately efficient at converting NOx into N2, and they are not efficient at converting CO and hydrocarbons into less harmful substances.
Another type of catalyst that removes NOx during lean conditions is the NOx storage catalyst, also referred to as a NOx trap or NOx adsorber catalyst. A NOx adsorber catalyst typically operates in two phases. First, during lean bum operation, nitrogen oxides, after being oxidized to NO2, are stored, for example, in the form of barium nitrate. Second, during rich operations, the nitrogen oxides are de-stored and treated. This type of catalyst relies upon the adsorption of NO2 onto the catalyst in the form of nitrates. However, under lean operating conditions, the NO2 portion of the exhaust relative to NO is small, NO2:NO is approximately 1:9, and NOx adsorber catalysts cannot effectively adsorb NO.
In order to improve the operation of the NOx adsorber catalyst, an oxidation function is built into the NOx storage catalyst, typically, in the form of a precious metal catalyst in order to oxidize NO to NO2. Additionally, an oxidation catalyst may be installed upstream of the NOx adsorber catalyst to further assist the conversion of NO to NO2.
When operating a NOx adsorber catalyst, one must periodically regenerate it. During regeneration, the stored nitrates are desorbed from the NOx adsorber catalyst in the form of NOx and transformed into nitrogen by reaction with the reductants in the exhaust gas. In many known processes, regeneration events take place approximately 10% of the time, and occur under rich operating conditions. During these regeneration processes, NO2 is reduced to N2 over the precious metals in the catalyst, as is typical in a traditional three-way catalyst.
Unfortunately, it is not easy to produce a rich environment in a lean burn engine. One known method is to run the lean burn engine under rich conditions by internal management alone. However, this method is complex and requires extensive engine recalibration. Further, it may prove particularly challenging in the case of heavy-duty diesel engines. Thus, it can be cumbersome and costly.
Another well-known method is to inject secondary reactants in the exhaust stream upstream of the NOx adsorber catalyst. By injecting secondary reactants at the appropriate place, one is able to achieve a rich environment without interfering with engine calibration.
For NOx adsorber catalysts, the preferred reductants are, in order of decreasing desirability based on effectiveness, H2>CO>hydrocarbons. However, it is impractical to store large quantities of H2 and CO in a vehicle. Consequently, practical constraints dictate that hydrocarbons stored in an automobile as fuel function as the reductant.
Due to the limitations of the aforementioned methods for regenerating NOx adsorber catalysts, there is a need to develop improved operating systems and methods for regeneration of these types of catalysts. As more stringent environmental regulations are adopted, this demand is increasing. The present invention provides a solution to regenerating NOx adsorber catalyst according to a method that will permit compliance with more stringent environmental regulations.